1. Field of the Invention
The invention relates to a plasma generator which generates a high-density plasma at low pressure, and a surface treatment apparatus which executes a surface treatment using this plasma generator with a substrate such as a semiconductor wafer.
2. Description of the Related Art
Apparatuses for executing a surface treatment of a substrate such as a semiconductor wafer or liquid crystal display (LCD) substrate using a plasma are well-known as plasma-enhanced chemical vapor deposition (PECVD) apparatuses, and dry etching apparatuses. Among those apparatuses, it is necessary to generate a high-density plasma to obtain higher treatment rates. It is also required to generate high-density plasmas at a lower pressure to prevent contamination of the substrate.
To improve ionizing efficiency in generating a high-density plasma at a lower pressure, helicon wave plasma generators, electron cyclotron resonance (ECR) plasma generators and inductively-coupled type radio-frequency (RF) plasma generators have been developed. As an example of those prior plasma generators, FIG. 10 shows a schematic front view of an inductively-coupled RF plasma generator disclosed in the Japanese unexamined publication No. H3-79025.
The plasma generator shown in FIG. 10 comprises plasma generation chamber 1 having dielectric window 11, RF coil 2 provided outside plasma generation chamber 1 and close to window 11, RF source 3 to supply a RF power with RF coil 2 through matching box 31.
Plasma generation chamber 1 is an air-tight chamber equipped with a pumping system (not shown). Plasma generating gas is introduced into plasma generation chamber 1 by a gas introducing system (not shown). Substrate 40 is held on the upper surface of substrate holder 4 which is provided at the lower side in plasma generation chamber 1. At a plane parallel to substrate 40 RF coil 2 is formed into a spiral having an axis vertical to the plane of substrate 40.
By the RF power supplied from RF source 3 an inductive electric field is applied in plasma generation chamber 1 through dielectric window 11. The plasma generating gas introduced into plasma generation chamber 1 changes into a plasma, forming a discharge by the inductive electric field. This plasma is called "inductively coupled plasma" (ICP) because RF coil 2 and the plasma are inductively coupled through dielectric window 11 when the plasma is generated.
So-called high-density plasma generators, as shown in FIG. 10, can generate a high-density plasma of the 10.sup.11 cm.sup.-3 at pressures of 10.sup.-3 Torr. However, those high-density plasma generators have a problem in that high-energy electrons, which adversely affect a surface treatment, may be produced simultaneously when high-density plasma is generated.
More specifically, for example, the SiO.sub.2 /Si selective etch process by plasma etching methods using reaction gas such as C.sub.4 F.sub.8 has been studied. This selective etch process utilizes a phenomenon in which the etching would be stopped on an Si layer where a carbon polymer film is deposited, while the etching would not be stopped on an SiO.sub.2 layer where no polymer film is deposited because oxygen in the layer produces volatile CO, CO.sub.2 and COF.sub.2.
Through the study of the inventors, it was found that in the SiO.sub.2 /Si selective etch process the selectivity of SiO.sub.2 /Si may decrease when the RF energy applied to the plasma increases. The reason for this is that the increased applied energy produces a large number of high-energy electrons, which may excessively dissociate a C.sub.4 F.sub.8 gas. Though the exact mechanism cannot be described, it is supposed that the Si layer would be etched because the carbon polymer film is deposited involving fluorine chemical species which are activated by the high-energy electrons, or the carbon polymer film is deposited under the existence of active fluorine or fluoride.
In anisotropic etchings, such as a reactive ion etching (RIE) where an electric field is applied vertically to a substrate for acceleration of reactive ions, the substrate is occasionally damaged by high-energy electrons. The high-energy electrons which cause such problems can be produced with high possibility when the high-density plasma generator shown in FIG. 10 is used. This is because the apparatus shown in FIG. 10 uses an RF of several MHz more than 10 MHz such as 13.56 MHz, where there is a high probability of electrons changing their moving directions following the alternating field inducted in plasma generation chamber 1. This is also a result of the low probability of electrons moving to the surface of plasma generation chamber 1 and losing their energy by collision with the surface.
Applying an RF greater than 0 MHz with RF coil 2 enables a reduction in the probability of electrons following the alternating field. But, it is very difficult to excite an RF greater than 100 MHz with RF coil 2 having the spiral shape shown in FIG. 10. Moreover, reducing the probability of electrons following the alternating field, which can restrain the production of high-energy electrons, may make it difficult to generate a high-density plasma, which is the merit of the apparatus.
On the other hand, ECR plasma generators using a microwave can generate a high-density plasma at a low pressure because electrons perform the cyclotron movement because of the effect of a magnetic field. However, the ECR condition requires a high magnetic field strength close to 1000 gauss, which may result in movement caused by the effect of a magnetic field. However, the ECR condition requires a high magnetic field strength close to 1000 gauss, which may cause a problem where a surface treatment loses uniformity by its influence. Charged particles, i.e., ions and electrons transferred along the magnetic lines easily result in a problem of the charge-up damage of the substrate, in which the substrate is damaged through the dielectric film breakdown caused by in-substrate-surface potential differences coming from the non-uniform charge-up because those charged particles may charge up the substrate in accordance with the magnetic profile on the substrate.
Meanwhile, as shown in the Japanese unexamined publication No. H7-307200, a technique of applying an RF in a plasma generation chamber by a radial antenna was proposed recently. In the study of the inventor, it was realized that high-density plasmas with low electron temperature can be generated by applying a RF from 100 MHz to 1 GHz by this type of antenna and this technique can be utilized with high-quality surface treatments without the excessive gas dissociation which may reduce the selectivity of SiO.sub.2 /Si.
However, problems described below were also found. First of all, in the composition where the antenna is provided in the plasma generation chamber as shown in the JUP No. H7-307200, the antenna is exposed to the plasma, resulting in a problem in that the antenna is sputtered. The sputtered material of the antenna reaches the substrate to cause contamination.
The inventors fabricated the antenna shown in the JUP No. H7-307200 and tried to generate a plasma, applying an RF with some antenna elements composing the radial antenna (for example, two rods 180 degrees apart from each other). Then, it was also found that generating a uniform plasma on a plane parallel to the plane where each antenna element is radially disposed (this plane is hereinafter called "antenna plane") is difficult because the generated plasma tends to be dense at the space beneath the antenna elements where the RF is applied. This tendency dose not change when the number of antenna elements where the RF is applied is increased. Even when the RF is applied with all antenna elements, the plasma tends to be dense at a specific space beneath a specific antenna element.